did you try enabling subtitles via plex on a 4k title?..that should start stutter

I'm playing 4K video with DTS audio and external subtitles from the TS-470 Pro on a LG TV with the Plex client app and it works fine. CPU is only around 20% because it doesn't need to do any transcoding. The first few minutes the CPU was around 96% but the NAS is also running a backup job at the same time and there was a lot of HDD activity.

However, I cannot play the same file on my iPad in the Plex app because the NAS's CPU is to weak to do the live transcoding.

If your iPAD has an A9 processor or better, then it should have h.265 HEVC decoding built in. You can use the inFuse app to play 4k content, it'll be decoded on the ipad so you NAS only needs to act as a normal file server.

If your iPad has an A8 or lower, then it can only do h.265 HEVC software decoding up to 1080p 20mbps with inFuse. But again, your NAS would need to act just as a normal file server.

If you're trying to play 4k content on an A8 or lower iPad, then you'll definitely need a fairly beefy processor to transcode 4k.